1. Field of the Invention
The present invention relates to a descaling device for slabs or strips in hot rolling mills. The descaling device includes a plurality of nozzles which are fed by a pressurized water piston pump and produce jets for the removal of scale adhering to the surfaces of the slabs or strips.
2. Description of the Related Art
When slabs or strips are descaled prior to hot rolling, a distinction is made between primary scale and secondary scale. Secondary scale is formed on already rolled material when the rolled material is heated to rolling temperature or is kept at rolling temperature. Primary scale is scale which is formed on the material after casting of slabs or strips. Primary scale adheres much more firmly to the material than secondary scale. If the cast material is continued to be heated for a longer period of time, even more scale is formed which adheres even more firmly to the material. In addition, the adherence of the scale strongly depends on the materials being used.
Slabs or strips to which secondary scale adheres are descaled by means of pressurized water which is sprayed through nozzles onto the surface of the slab or strip. As a rule, the pressurized water is pumped by centrifugal pumps which work at maximum pressures of 300 bars. The resulting specific impact force and the quantity of water determine the descaling effect. Satisfactory results are achieved when secondary scale is removed by means of this arrangement.
However, slabs and strips are increasingly cast and hot rolled from the casting heat, wherein an equalizing furnace may be interposed. Therefore, it is necessary to remove primary scale which adheres to the material. The descaling effect depends on the quantity of water and the pressure which, among other things, determine the impact force.
When steel is continuously cast and subsequently hot rolled from the casting temperature, the entry speed at the first hot rolling stand is approximately 1/3 of the speed of the already rolled material. When the quantity of water per unit of time remains the same, these speed differences cause the cast material to be cooled, as compared to the pre-rolled material, to such an extent that the temperature of the material drops below the minimum temperature for the hot rolling procedure. Even if the quantity of water per unit of time is converted to the slower entry speed, an increase of the quantity of water is hardly possible in cast materials, because the material would be cooled more strongly because of the slower entry speed and an increase of the quantity of water would result in excessively strong cooling.
This means that the impact force of the water can only be increased by increasing the pressure. Since this requires pressures of up to 600 bars, the centrifugal pumps have to be replaced by piston pumps. However, it was found that the descaling device is subjected to substantial wear at such high pressures.